1. Field of the Invention
The present invention relates to a process for producing zinc or zinc alloy powder for batteries. More particularly, it relates to a process for producing zinc or zinc alloy powder for batteries as a well-balanced mixture of fine powder and coarse powder which exhibits satisfactory workability in gelation and acquires satisfactory gel viscosity, ensures excellent battery characteristics with reduced gas evolution when used in batteries, and can be produced in large quantity in good yield.
2. Description of Related Art
Zinc or zinc alloy powder for use in batteries, especially as an anode active material of alkaline batteries, has been produced by gas atomization in which molten zinc or zinc alloy is atomized by an air jet or an inert gas jet.
In carrying out gas atomization, zinc or a zinc alloy material is melted in a melting furnace, and a predetermined amount of the molten metal is dropped in a stream. The stream of falling molten metal droplets is struck at right angles by an atomizing gas jet emitted from a nozzle exit and broken up into particles, which are collected. The orifice of the nozzle usually used in gas atomization has a circular or annular cross-section. Air has been frequently used as an atomizing gas. Zinc or zinc alloy powders produced by the gas atomization generally comprise 20 to 200 mesh particles in a proportion of about 75% by weight.
With the recent rapid development of digitization, the demand for higher-rate alkaline batteries has been growing, and further improvement on activation of zinc or zinc alloy powder by particle size reduction has been required accordingly. On the other hand, it is required for the zinc or zinc alloy powder to contain coarse particles greater than a certain size from the standpoint of workability in gelation, the resultant gel viscosity, and suppression of gas evolution within batteries. For these reasons, it has been demanded to establish a technique suited to commercial production of a zinc or zinc alloy powder comprising coarse particles and fine particles in a given ratio and satisfying the performance requirements described above.
Zinc or zinc alloy powders having a prescribed particle size distribution have been obtained by classifying gas-atomized powders into fractions according to size and mixing the fractions. However such classification and mixing operations are extremely inefficient, contributing to poor yield and poor productivity.
An object of the present invention is to provide a process of producing a zinc or zinc alloy powder for batteries which can yield a powder containing fine particles and coarse particles in arbitrary amounts in a good balance, exhibiting satisfactory workability in gelation and satisfactory gel viscosity, and, in batteries, manifesting excellent battery characteristics and suppressing gas evolution and which realizes large volume production with good yield.
As a result of extensive investigation, the present inventors have found that the above object is accomplished by carrying out a gas atomizing process for producing a zinc or zinc alloy powder by using a plurality of nozzles arranged in parallel with each other and optimizing the molten metal flow rate and the atomizing pressure for each nozzle.
Completed based on the above finding, the present invention provides a process of producing a zinc or zinc alloy powder for batteries which comprises dropping molten zinc or a molten zinc alloy to form a stream of molten metal droplets and striking a jet of an atomizing medium emitted from the orifice of a nozzle against the molten metal stream at right angles to atomize the molten zinc or the molten zinc alloy, wherein
two or more the nozzles are arranged in parallel to each other, the orifice of each of the nozzles has a V-shaped, U-shaped, X-shaped or arc-shaped cross-section, the atomizing medium is air or an inert gas,
two or more the molten metal streams have at least two different flow rates selected from a range of from 0.04 to 0.25 kg/sec, and two or more the jets of the atomizing medium have at least two different atomizing pressures selected from a range of from 4 to 9 kg/cm2.